Preforms, from which step index and graded index optical fibers are drawn, can be manufactured in any one of a variety of ways. One such way is the so-called "modified chemical vapor deposition" (MCVD) process in which a plurality of layers of suitably doped silicate glasses are deposited on the inside of a fused silica tube. After the requisite number of layers (i.e., typically about 50), have been deposited, the tube is collapsed into a solid rod which constitutes the preform from which a fiber can be drawn. For a more detailed discussion of the MCVD process, see the article by J. B. MacChesney, Chesney, P. B. O'Connor and H. M. Presby entitled "A New Technique for the Preparation of Low-Loss and Graded-Index Optical Fibers," published in the Proceedings of the IEEE, pp. 1278-1279. Also see, "Preparation and Structural Characteristics of High Silica, Graded Index Fibers" by P. B. O'Connor, J. B. MacChesney, H. M. Presby and L. G. Cohen, published in The American Ceramic Society Bulletin, Vol. 55, No. 5, May 1976, pp. 513-519.
After the preform is made, the fiber is drawn and tested. While it would be reasonable to assume that fibers drawn from preforms fabricated in substantially the same manner would exhibit similar characteristics, this has not always been the case. Indeed, it is not unusual for lengths of fibers drawn from different portions of the same preform to have significantly different transmission characteristics. Unfortunately, some lengths of fibers are totally unusuable and must be discarded.
While the ultimate cost of mass produced optical fibers can only be guessed at this time, it is known that the cost of pulling the fiber from the preform, coating it and then testing it might be anywhere between 30 and 50 percent of the total cost of the fiber. It is apparent, therefore, that a significant saving could be realized if there was some simple and convenient way of examining the preform before the fiber is drawn to determine whether or not a usuable fiber is likely to be produced.
Techniques for inspecting glass rods for gross defects are well known. In U.S. Pat. Nos. 3,777,171 glass tubing is inspected for flaws by directing a pair of narrow band, high intensity laser beams, spaced 90 degrees from each other about the axis of the tubing, through the tubing. Changes in the intensity of the transmitted light are detected by a pair of photodetectors, thus providing an indication of the presence and location of a flaw.
U.S. Pat. No. 4,021,217 discloses a method of inspecting optical fibers for surface cracks. In particular, light that is scattered out of the plane normal to the fiber axis is monitored by means of a photodetector. When defects are detected, the fiber drawing process is terminated.
Neither of these patents, however, is concerned with, or discloses means for examining the internal structure of a glass rod and, in particular, of optical fiber preforms.